Early investigations utilizing atrial pacing in man to study the variations in scalar ECG morphology and vector loop of the P wave with varying sites of focal atrial origin, were primarily geared towards the differentiation of left- from right-sided atrial rhythms (e.g. Massumi R. A. and Tawakkol A. A. (1967), Direct study of left atrial P waves, Am. J. Cardiol. 20:331-340; and Harris B. C. et al. (1968) Left atrial rhythm; Experimental production in man, Circulation 37:1000-1014). Despite several attempts to develop a set of morphologic ECG criteria specific to a left-sided origin of an ectopic atrial rhythm, different algorithms were proposed and consensus appeared difficult to attain.
Mirowski in 1967 initially suggested that a negative P wave polarity in lead I and a “dart and dome” configuration in V1 was related to a left-sided origin and later added that a negative P wave in V6 was more specific particularly when the aforementioned features were absent (Mirowski M. (1967) Ectopic rhythms originating anteriorly in the left atrium; Analysis of 12 cases with P wave inversion in all precordial leads, Am. Heart J. 74:299-308). Although these findings were partly underlined by others (e.g. Leon D. F. et al. (1970) Right atrial ectopic rhythms; Experimental production in man, Am. J. Cardiol. 25:6-10), Harris et al. in 1968 (same reference as mentioned above) contested the importance of P wave inversion in leads I and V6 for a left-sided rhythm and alternatively proposed that a terminally positive P wave in V1 was a more specific finding for a left atrial origin. Conversely, Massumi and Tawakkol (same reference as mentioned above) noted a profound variability in P wave morphology after electrically stimulating the left atrium from similar anatomical sites in different patients and did not believe that distinct ECG criteria specific to areas of ectopic left atrial impulse formation could be developed.
Using temporarily implanted pacing wires following cardiac surgery, Maclean et al. subsequently performed a comprehensive study in 69 patients by stimulating both atria at a total of 12 epicardial regions (Maclean W. A. H. et al. (1975) P waves during ectopic atrial rhythms in man; A study utilizing atrial pacing with fixed electrodes, Circulation 52:426-434). Overall the results of this study were disappointing in that only a few specific correlations between P wave morphology and site of origin could be made: (1) a negative P wave in the inferior leads with pacing of the inferior regions in either atrium; (2) a negative P wave in lead I with left atrial pacing near the left pulmonary veins; (3) a positive bifid P wave in V1 with pacing near the lower pulmonary veins and coronary sinus. Therefore, these investigators and recently others using contemporary multisite endocardial catheter mapping techniques (Man K. C. et al. (1996) Spatial resolution of atrial pace mapping as determined by uniploar atrial pacing at adjacent sites, Circulation 94:1357-1363), underline the complexity of visual analysis of the low-voltage P wave and concluded that the 12-lead ECG was of limited clinical value in localizing ectopic atrial foci.
The limited role of the standard 12-lead ECG to distinguish right from left atrial focal activity has also been reported by Kalman et al. during ablation of atrial tachycardia guided by intracardiac echocardiography (Kalman J. M. et al. (1998) “Cristal tachycardias”: origin of right atrial tachycardias from the crista terminalis identified by intracardiac echocardiography, J. Am. Coll. Cardiol. 31:451-459). Kalman et al. reported that the P wave morphology on the 12-lead ECG of tachycardias arising from the upper part of the crista terminalis in the right atrium or the right upper pulmonary vein in the left atrium, demonstrates considerable overlap due to the proximity of both structures. Tang et al. suggested previously that a change in P wave morphology in lead V1 from biphasic during sinus rhythm to completely positive during tachycardia would be helpful in discriminating foci arising from the superior portion of the crista terminalis and the right upper pulmonary vein (Tang C. W. et al. (1995), Use of P wave configuration during atrial tachycardia to predict site of origin, J. Am. Coll. Cardiol. 26:1315-1324). The latter finding was found to be predictive for right upper pulmonary vein tachycardia foci even when lead aVL would demonstrate a positive instead of a negative P wave.
There have also been attempts to localize the atrial insertion site of an accessory pathway during orthodromic atrioventricular (AV) reentrant tachycardia. Farshidi et al. initially demonstrated that a negative retrograde P wave in lead I was associated with a left-sided atrial insertion of the accessory pathway (Farshidi A. et al. (1978) Electrophysiologic characteristics of concealed bypass tracts: clinical and electrocardiographic correlates, Am. J. Cardiol. 41:1052-1060). Garcia Rivera et al. was able to separate free wall accessory pathway locations in the left and right atrium using the retrograde P wave polarity in leads I and V1 (Garcia Rivera R. et al. (1980) Retrograde P wave polarity in reciprocating tachycardia utilizing lateral bypass tracts, Eur. Heart J. 1:137-145). Other investigators have later studied the localization resolution of the 12-lead ECG for this particular application by confining pace mapping to the annular regions of the left and right atrium (e.g. Fitzgerald D. M. et al. (1996) P wave morphology during atrial pacing along the atrioventricular ring; ECG localization of the site of origin of retrograde atrial activation, J. Electrocardiol. 29:1-10). However, despite a more directed pace mapping approach, the paced P wave morphology only allowed a gross separation of pacing sites in terms of a left-versus right-sided origin, an inferior origin in either atrium, or an origin in the right free wall. In contrast, Tai et al. recently reported that the polarity of the retrograde P wave in leads I, II, III, aVF, and V1 obtained during AV reentrant tachycardia allows accessory pathway localization to 9 possible annular regions with an overall accuracy of 88% provided a clearly visible P wave could be discriminated (Tai C. T. et al. (1997) A new electrocardiographic algorithm using retrograde P waves for differentiating atrioventricular node reentrant tachycardia from atrioventricular reciprocating tachycardia mediated by concealed accessory pathway, J. Am. Coll. Cardiol. 29:394-402). It has also been realized with this application of the surface ECG, that visual analysis of the low-voltage retrograde P wave is frequently hampered by the simultaneous occurrence of the preceding cardiac cycle's high-voltage TU wave.
As mentioned above, visual inspection of the 12-lead ECG in an attempt to correlate changes in P wave morphology and polarity with various locations of ectopic activity is hampered by a low-amplitude signal which is often buried in the preceding TU wave, and also by inter-patient inconsistency in P wave pattern with comparable locations of ectopic left atrial activation, and poor overall spatial resolution in both the left and right atrium. It has been recently demonstrated using right atrial pace mapping that a significantly higher electrocardiographic localization resolution is clinically feasible when multi-lead ECG techniques are adopted (SippensGroenewegen A. et al. (1998) Body surface mapping during pacing at multiple sites in the human atrium; P wave morphology of ectopic right atrial activation, Circulation 97:369-380).
There is one preliminary clinical report investigating the use of multi-lead ECG or body surface mapping to discriminate ectopic left atrial activation (Kawano S. and Hiraoka M. P. (1995) P wave mapping in ectopic atrial rhythm. In: Yasui S. et al. Eds. “Advances in Body Surface Mapping and High Resolution ECG”: Nagoya, Japan: Life Medicom pp. 47-56). Kawano and Hiraoka showed that endocardial pacing at one right (low) and three left (low, middle, and high) atrial locations produced characteristic P wave body surface potential map patterns. Access to the latter left atrial locations was attained exclusively through the coronary sinus. However, only a moderate number of pacing site locations were studied while pacing from the pulmonary veins was not attempted.
It is clear that the development of noninvasive methods for clinical localization of left atrial arrhythmias with sufficient and high electrocardiographic resolution is difficult. Therefore, noninvasive localization of left atrial arrhythmias is in need of alternative methods that enable improved discrimination and differentiation of electrocardiographic P wave patterns or morphologies.